3GPP LTE is the largest new technology R&D project launched by 3GPP in recent two years. Because, in 3GPP LTE, downlink transmission technology utilizes new Physical Layer Transmission technology, i.e. OFDM, E-MBMS under this circumstance has many brand new characters compared to MBMS in WCDMA Release6 Protocol.
In 3GPP LTE, downlink transmission scheme is based on conventional OFDM using a cyclic prefix (CP), with a sub-carrier spacing Δf=15 kHz and a cyclic-prefix (CP) duration Tcp≈4.7/16.7 μs (short/long CP). Long CP aims to be applied to multi-cell MBMS transmission and macro-cell environment with long inter-site distance.
MBMS transmission may be performed in the following two ways: one is multi-cell transmission and the other is single-cell transmission. In case of multi-cell transmission, the cells and contents are synchronized to enable the user equipment (UE) to combine the energy from multiple radio links without additional receiver complexity. Said combining is coherent combining, which is also called RF combining. RF combining requires all signals to arrive at UE receiver within the window defined by CP, therefore tight inter-cell synchronization is needed. In addition, considering the reference (pilot) signal combining, under condition of reference signals are designed as common pilot signals of the cell, since the MBMS reference signals of all cells are identical, RF combining of signals can be done.
However, in cases of asynchronism between cells, or in bad urban environment with large multi-path time delay spread, signals arriving UE receiver may be out of CP window and these signals will lead to interference between symbols. Moreover, because pilot and data of broadcast service are identical, signals are strong coherence. If these signals arriving are not within CP window, interference between cells will affect pilot estimation and data receiving, which great attention must be paid to.
In 3GPP LTE, it proposes that downlink reference signal(s) can be used for:                downlink-channel-quality measurements        downlink channel estimation for coherent demodulation/detection at the UE        cell search and initial acquisition        
The basic reference signal structure discussed in 3GPP LTE is illustrated in FIG. 1. As seen in FIG. 1, reference symbol, which is also called the first reference symbol, locates in the first OFDM symbol position of the sub-carrier which is allocated to downlink data transmission. Additional reference symbol, which is also called the second reference symbol, locates in the fifth OFDM symbol position of the sub-carrier which is allocated to downlink data transmission. In implementation, MBMS transmission, reference symbol and additional reference symbol could be different and have different repeat frequency.
Based on aforesaid structure, there are several existing methods for sending MBMS data in 3GPP LTE. In these methods, MBMS data are channel coded and modulated, and then scrambling code is selected. The selected scrambling code could be a common scrambling code for all cells, or a dedicated scrambling code for each cell. Based on the selected scrambling code, pilot and/or date are scrambled with the selected scrambling code and then sent. The scrambled pilot and data structures are illustrated in FIGS. 2A, 2B and 2C.
FIGS. 2A, 2B and 2C respectively show data structures scrambled with different scrambling codes, wherein, abscissa represents time domain, ordinate represents frequency domain, and each rectangle grid represents a symbol. FIG. 2A shows pilot and data structures generated by scrambling pilot with a common scrambling code of the cells. FIG. 2B shows pilot and data structures generated by scrambling pilot with a common scrambling code of cells and a cell-specific scrambling code. FIG. 2C shows pilot and data structures generated by scrambling pilot with a cell-specific scrambling code and scrambling different MBMS traffic data with service scrambling code.
Though aforesaid existing data sending methods could scramble pilot and data, they are on the premise of tight synchronization of all E-nodeBs. In this case, signals of surrounding cells could arrive at the UE receiver within the CP window. Even though there are signals leaked out of CP window, it is considered that the interference impact of their power on the receiver could be neglected.
However, in cases of inter-cell asychronism, or in bad urban environment with large multi-path time delay spread, signals arriving at the UE receiver may be out of the CP window. Moreover, because pilot and content of broadcast service are identical, signals are strongly coherent so that the inter-cell interference will affect the pilot estimation and data receiving.
To solve the problem that signals arriving at the UE receiver not within CP window due to inter-cell asychronism may cause interference, the applicant proposes a technical solution of dividing multiple cells into cell groups on basis of considering the E-nodeB synchronization, downlink macro diversity and UE receiving combining in E-UTRAN. Wherein, a scheme of cell groups division in E-UTRAN is proposed. In this scheme, it divides all cells into multiple MBMS cell groups according to the time delay of radio propagation. Each cell group contains several E-nodeBs and their cells/sectors. The diameter of cell group coverage is equal to or slightly less than the distance with radio propagation in long CP window, that is to say, when a UE locates in a cell group, all signals transmitted from E-NodeBs in this cell group should arrive at UE within the CP window.
To transmit MBMS on the basis of above MBMS cell group division, method of sending and receiving data as well as corresponding device and system adapted for the scheme of said MBMS cell group division are needed.
As the evolution of MBMS in 3GPP, E-MBMS utilizes new Physical Layer Transmission technology, i.e. OFDM, which is quite different from that of MBMS. It is the different Physical Layer Transmission technology that makes E-MBMS has many brand new characters as compared to MBMS defined in WCDMA Release6 Protocol.
In 3GPP LTE, downlink transmission scheme is based on conventional OFDM using a cyclic prefix (CP), with a sub-carrier spacing Δf=15 kHz and a cyclic-prefix (CP) duration TCP≈4.7/16.7 μs (short/long CP). Wherein, long CP aims to be applied to multi-cell MBMS transmission and macro-cell environment with long inter-site distance. In most cases, multicast traffic and unicast traffic are transmitted by time domain multiplexed (TDM), or on the separate carrier respectively. Here, multicast service utilizes a single long CP length. In the cases of transmitting multicast traffic and unicast traffic by frequency domain multiplexed (FDM), the long CP is also required to meet the need of multicast traffic with precedence.
In synchronous E-UTRAN system, since signals arrive at the UE receiver within CP window, the inter-cell interference will not cause big problems. Because the time delay spread of signals within CP window will not cause inter-symbol interference (ISI) of OFDM symbols, these time delay spread signals could be combined in frequent domain after FFT processing. However, in other cases, interference from surrounding cells is beyond the bound of CP window. Because the contents of broadcast traffic are identical, their signals are strongly coherent. Signals out of the CP window will cause severe ISI. The network with this problem could have got:                inter-cell asynchronism;        large multi-path time delay (e.g. BU: bad urban environment)        
In asynchronous system synchronized by physical layer synchronization technology, timing reference of different NodeBs are independent, and there could a certain time drift between cells. In 3GPP UMTS, the frame structure of the cells could be slowly sliding relative to each other. The absolute accuracy requirement of the drifting on the timing reference of a cell must be less than ±0.05 ppm.
With that accuracy, the relative time drift between two cells could be as much as a long CP window length (16.7 μs) per 3 minutes. Thus, the inter-NodeB synchronization within whole E-NodeBs in the service area of MBMS shall be performed once every 3 minutes, this would be too complex and inefficient. Further, the re-synchronization procedure would be too frequent.
Assuming all of the E-NodeBs are synchronous, that is to say, the system is synchronous system. The signals with same content transferred from surrounding cells don't always arrive within the window defined by CP when the radio propagation is in the Bad Urban environment and with inter-site distance of 1732 m. Wherein, BU is a typical urban channel environment in the COST 207 model adopted by 3GPP LTE, and inter-site distance of 1732 m between NodeBs is a bad macro-cell configuration mode adopted by 3GPP LTE. According to the COST 207 model, in bad urban environment, the fifth path is 5 μs after the main path and has −2 dB mean power, and the sixth path is 6.6 μs after the main path and has −4 dB mean power. Therefore, even in synchronous system, it is probable that considerable power is leaked out of the CP windows which would turn to the inter-symbol interference (ISI)
However, it is assumed that all E-NodeBs are strictly synchronous in the prior art (actually it isn't), and signals from any NodeB in a wide range could arrive at UE within the CP window. Even though they don't arrive within CP window, it is considered that the impact of the power on UE could be neglected. It is obvious that because the assumed scene couldn't be satisfied in practice, the signals arriving outside the CP window will affect the pilot estimation and data receiving at the receiver.
Therefore, a method and device are required to avoid the signals arriving out of the CP window from causing ISI.